This proposal will test the hypothesis that platelet membrane glycoprotein GPIIb-IIIa is a member of the cytoadhesin family of structurally, immunochemically and functionally related membrane proteins. The involvement of platelets in thrombotic diseases and in hemostasis is dependent upon their adhesive properties. Platelet adhesive reactions are regulated by their interaction with a set of adhesive proteins, which includes fibrinogen, fibronectin and von Willebrand Factor. GPIIb-IIIa serves as a receptor for these adhesive proteins and has a recognition specificity for the Arg-Gyl-Asp sequence. This sequence occurs in the three platelet adhesive proteins, and Arg- Gly-Asp containing peptides inhibit the binding of these proteins to platelets. GPIIb-IIIa is not restricted to platelets as immunologically related molecules have now been identified on endothelial cells, fibroblasts, smooth muscle cells, erythoroleukemic cells and monocytoid cells. These widely distributed cell surface molecules share the gross structural properties of being comprised of two similar subunits, one of approximately 100 kD and the second of approximately 140 kD. At least two of these molecules, GPIIb-IIIa and the vitronectin receptor, are Arg-Gly-Asp receptors. Based upon these lines of evidence, the existence of the cytoadhesin family and the membership of GPIIb-IIIa in this family is postulated, and this hypothesis will be explored. The structural and immunochemical relationship among the cytoadhesins will be examined to determine if their subunits are comprised of common and cell- specific regions. This will be accomplished by developing and characterizing the reactivity of a series of anti-peptide antibody reagents. In addition, the immunological relationship between GPIIb-IIIa and the endothelial cell cytoadhesin will be explored in detail. The primary structure of GPIIb-IIIa will be deduced by gene cloning approaches, and the distribution of mRNA for the cytoadnesins in cells and in tissues will be established. A functional linkage among the cytoadhesins will be sought by assessing their role as Arg-Gly-Asp receptors and their capacity to bind adhesive proteins. Finally, specific roles of cytoadhesins in selected cellular functions will be evaluated. The role of vitronectin as a platelet adhesive protein and the contribution of the cytoadhesins and the Arg-Gly-Asp recognition specificity in platelets-monocytes and neutrophil-endothelial cell interactions will be evaluated. On an overall basis, these studies should provide fundamental insight into the mechanisms underlying cell adhesion.